The invention relates to a capacitive proximity switch for the evaluation of capacitance changes with an electrical alternating current measurement bridge of four bridge branches, which reach every two of four switching knots of the bridge, wherein an alternating current is positioned as a bridge feed voltage near two opposite, not neighboring switching knots, and respectively the two bridge branches between the two feed in switching knots build each of the two halves of the bridge, and there is an evaluated bridge diagonal voltage on the two remaining switching knots over diagonal voltage path, wherein the bridge diagonal voltage is removable, for detecting an approaching or moving away face, in particular an electrically bad conductive face or metallic face, which face is part of variable capacitor in one of the bridge branches of the bridge circuit, wherein at least one capacitor (capacitive bridge) or at least one capacitor and/or one resistor (capacitive ohmic bridge) is disposed in the remaining bridge branches as further reactences, wherein rectifiers are arranged to the rectification of the both bridge branch voltages separately according to respective bridge halves in the diagonal voltage paths and the diagonal bridge voltage is evaluated only after the rectification of the both bridge branch voltages as changing direct current corresponding to the capacitance change of the variable capacitor, according to the preamble of the claim one as well as method according to preamble of the claim 9.
The evaluation of small up to very small changes of capacitance is an always returning task in sensor technology. Very small changes in capacitance, namely in an order of magnitude of less than 10 fF, have to be reliably evaluated in particular in connection with capacitive proximity switches, wherein here in particular the stability against interferences as well as the temperature stability of the respective circuit assumes a central importance.
It is known to employ an oscillator for determining small changes in capacitance, wherein the oscillating amplitude of oscillator changes depending on the capacitance of the sensor. The size of the oscillation amplitude thus is a measure for the value of the capacitance of the sensor. Such a circuit is associated with the disadvantage that the circuit is not stable relative to temperature based on a principle, and therefore an eventually difficult to dimension temperature compensation is necessary in connection with such a circuit in most cases. Furthermore the quality and efficiency of the oscillator is low, wherefrom a broad band circuit with a bad electromagnetic compatibility behavior results.
Furthermore, it is known for the determination of small changes in capacitance to employ an oscillator, wherein the frequency of the oscillator changes depending on the capacity of the sensor. Again the disadvantage is associated with the employment of such an oscillator, that such a circuit is not stable relative to temperature based on a principle, and therefore here again an eventually difficult to dimension temperature compensation is necessary; also such circuit has a relatively bad electromagnetic compatibility behavior. so-called switched capacitor technique is furthermore known for determining small changes in capacitance, wherein a critical timing of the clock cycle signals is of disadvantage and wherefore an extremely stable clock cycle signal is required, which imposes an expensive switching technology depending on the method.
Also integrated circuits are furthermore known for the determination of small changes in capacitance. The integrated circuits exhibit the disadvantage that in most cases a ground free capacitance is necessary. In addition such circuits require in general a digital evaluation unit (mostly a counter) which means a large expenditure for switching technology. Such concepts are employed predominantly in the micro systems technology for these reasons.
It is furthermore known that the determination of small changes of an electrical value can be realized advantageously with the bridge circuits. The value to be measured is here compared with reference values, wherein these reference values are generated by similarly operating elements. Thus temperature influences can be effectively suppressed as long as the two bridge branches of the bridge have at all times the same temperature. Changes of the respective value are then presented as changes of the bridge diagonal voltage. The employment of reactances as bridge elements causes that the bridge has to be operated with alternating current. Thus also the bridge diagonal voltage represents an alternating voltage. The evaluation of the bridge diagonal voltage presents here frequently a problem according to the state of the art, because
the amplitude of the alternating voltage is very small based on the small change in capacitance, namely a few millivolts mV;
the frequency of the alternating voltage, which is employed for operating the bridge, and therewith also the frequency of the bridge diagonal voltage is located in the MHz region in order to avoid that the branch currents do not assume too small values given the small capacity values;
the bridge diagonal voltage in many cases in addition to the alternating voltage component also includes a common mode direct current component with a substantial larger as compared to the alternating current component.
Such previously known solutions are contained for example in the printed documents DE-C2-3143114, DE-A1-3911009, DE-A1-19536198, DE-A1-19701899, CH-558534, as well as in the EP-A1-0723166.
A circuit arrangement for eliminating the influence of a phase shift between the voltage potentials of the two measurement points of an alternating current measurement bridge with complex resistances is furnished, wherein the difference of the voltage potentials forms the measurement signal. A rectifier valve is disposed between the measurement points and the input enclosures of a circuit for forming the difference in each voltage path, wherein a storage disposed between the output of the rectifier valve and a reference potential common to the alternating current measurement bridge is connected following to each of the rectifier valves.
Capacitors employed as capacitive measurement value receivers within an alternating current measurement bridge, either in a bridge circuit with four capacitances or with two capacitances and two resistors, wherein in each case a capacitance is variable, are known from the literature location Heinz Schneider, Kondensatoren als Messwertaufnehmer (Capacitors as measurement value receivers), Elektronik-Applikation Nr. 14, Jul. 9, 1985.
A proximity switch with an alternating current measurement bridge is known by FR-A-2371676, which is subjected to an alternating current. The bridge branch voltages are rectified and subject to single electrical manipulations. Only after the rectification of the bridge branch voltages these as changing direct current as capacitive size are evaluated.
It is an object of the present invention to furnish a capacitive proximity switch as well as a method, which is highly sensitive to the approach of the object on one of its sides and by this allow a reliable evaluation of small changes in capacitance, wherein the circuit is to show a high resistance to interferences as well as a high-temperature stability based on its principle. Furthermore, proximity switch is to be realizable with comparatively small expenditure and therefore at low costs.
The proximity switch according to the present invention comprises a flat multilayer circuit board comprising at least two electrically insulating layers, wherein an electrically conductingly intermediate player is disposed between the two electrically insulating layers as a first place of capacitance in one of the two bridge branches of one of the bridge halves and wherein a flat electrically conducting covering is placed on one of the two layers furnishing a sensor, wherein the flat covering forms the second face of the capacitor, wherein the face is disposed movable relative to the sensor and forms with the sensor a second flat variable capacitor, and wherein the face and the sensor form one capacitance and wherein the sensor and the electrically conducting intermediate layer form the second capacitance of one of the two bridge halves and wherein this construction represents the one half of the bridge, and wherein rectifires are disposed in the diagonal voltage paths for rectifying the two bridge branch voltages separately according to the respective bridge half and wherein the bridge diagonal voltage is evaluated only after the rectification of the two bridge branch voltages as the direct current changing corresponding to the capacitance change of the variable capacitor.
The proximity switch and the method are associated with the advantage that a reliable evaluation of very small capacitance changes is possible, wherein the circuit exhibits a high stability against interferences and a high-temperature stability and wherein the circuit is substantially insensitive relative to coupled in interferences. Similarly the proximity switch can be realized with comparatively small expenditures. The advantages comprise in particular:
no alternating voltage has to be evaluated but only a direct voltage.
a slow operational amplifier or, respectively, comparator can be employed for evaluating the bridge diagonal voltage.
the rectification of the bridge branch voltages can be performed with diodes, whereby the very simple circuit with only a few device elements results; advantageously a so-called slow diode can be employed in order to suppress interferences. Alternatively rectification is performed synchronous with controlled switches, in case a particularly high suppression of interferences is required.
It is important that the rectifiers of the two branches exhibit the same temperature behavior in order for the rectified bridge diagonal voltage to be independent of temperature. It is advantageous for good suppression of interferences that the switching construction is furnished symmetrical, because an interference operating in the same way onto the two bridge branches does not cause the difference voltage at the rectifier outputs.
The rectification of the bridge branch voltages can be performed either by diodes, preferably four diodes, or by controlled switches, preferably four controlled switches, wherein in case of the employment of switches these switches are controlled pairwise in opposite phase and are switched synchronously with the bridge voltage from one switching state to the other.
In addition coupled in interferences can be suppressed effectively, by connecting and switching in each case a low pass filter, a LP-filter, in front of the rectifier of the bridge branch voltages. It is to be observed that the capacitance of the LP-filter can be formed by parasitic capacitances of the rectifier elements, for example barrier layer capacitances or, respectively, diffusion capacitances of the PN-transitions of rectifier diodes for or of the input capacitance of electronic switches, such that also the capacitors can be dispensed with. The electromagnetic compatibility behavior can be improved by the employment of relatively slow rectifier diodes. In this case possibly an additional low pass LP-filtering can be dispensed with.
Also four switches with pairwise opposite phase driving for rectifying the bridge branch voltages can be employed instead of the four diodes. It is thereby possible to rectify also alternating voltages, wherein the amplitude of the alternating voltages is smaller as compared with the threshold voltage of a rectifier diode. Furthermore, interfering voltages are better suppressed because only such alternating voltages are completely rectified, where the alternating voltages are synchronized with the switching signal of the switch. In order to decrease the probability that the interfering signal exhibits in the same frequency as the switching signal of the switch, it is recomended to change continuously the frequency of the bridge voltage and thereby also the frequency of the change signal.
Also two transfer switches can be employed instead of the four individual driven switches with pairwise opposite phase driving.
The face approaching or moving away, which is part of a capacitor of the capacitive proximity switch, can also be grounded.
According to the method, the two bridge branch voltages are rectified separately according to the respective bridge half either by four diodes or by four controlled switches as rectifires in the diagonal voltage paths, wherein the bridge diagonal voltage is driven synchronous with the bridge feed voltage pairwise in opposite phase upon employment of switches which are opened and closed synchronously with the bridge feed voltage pairwise by means of control voltage as well as opposite phase controlled and are switched synchronously with the bridge feed voltage from one switching state into the other switching state. The frequency of the bridge feed voltage and also of the transfer switch signal can be changed continuously.
The two bridge branch voltages are rectified separately according to the respective bridge half either by four diodes or by four controlled switches as rectifires in the diagonal voltage paths for evaluating small changes of capacitance under employing of a capacitive proximity switch, wherein the bridge diagonal voltage is evaluated only after the rectification of the two bridge branch voltages as a direct voltage changing corresponding to the change of the capacitance, and wherein the bridge diagonal voltage is driven synchronous with the bridge feed voltage pairwise in opposite phase upon employment of switches and wherein the bridge diagonal voltage is switched from one switching state into the other switching state synchronous with the bridge voltage. The frequency of the bridge feed voltage and also of the transfer switch signal can be changed continuously.
A balancing of the circuit or, respectively, of the proximity switch is advantageously performed by changing the capacitance of one of the capacitors in one of the bridge branches of the bridge. This can be performed with the aid of a so-called variable tuning capacitor or of a laser trimmed capacitor. It is advantageous to perform the adjustment and balancing such that the difference voltage is equal to zero in the switching point of the proximity switch, since it is then sufficient to evaluate only the sign of the difference voltage. If only the sign of the difference voltage Ud is evaluated, then there results an output signal with the two different states, wherein the switching point at which the sign of the difference voltage Ud changes from one state into the other depends only on the capacitance value of the variable tuning capacitor, however, not on the amplitude or the frequency of the bridge voltage ubr or on the size of the forward flow voltage Uf of the rectifier diodes.
If it is intended to dispense with the balancing with the aid of a variable tuning capacitor, then the zero point of the difference voltage can also be set by having two of the rectifier elements with their one connection not connected to the reference potential, for example ground, but in each case to a reference voltage source, wherein the value of the reference voltage source is set such that the desired difference voltage Ud, that is in most cases zero, is set at the output. If the two reference voltages are derived such from the bridge supply voltage ubr, that a linear connection exists between the respective reference voltage and the bridge supply voltage ubr, then at change of the bridge supply voltage ubr does not affect the bridge diagonal voltage Udxe2x80x94and thus the balancingxe2x80x94.